<template>
    <div class="list">
        <li 
            class="item" 
            v-for="item of letters" 
            :key="item" 
            :ref="item"
            @click="handleLetterClick"
            @touchstart.prevent="handleTouchStart"
            @touchmove="handleTouchMove"
            @touchend="handleTouchEnd"
        >{{item}}</li>
    </div>
</template>
<script>
export default {
  name: 'CityAlphabet',
  props: {
      cities: Object
  },
  data(){
      return {
          touchStatus: false,
          startY: 0,
          timer: null
      }
  },
  updated(){     
      this.startY = this.$refs['A'][0].offsetTop
  },
  computed: {
      letters () {
          const letters = []
          for(let i in this.cities){
              letters.push(i)
          }
          return letters
      }
  },
  methods:{
      handleLetterClick (e) {
          this.$emit('change', e.target.innerText)
      },
      handleTouchStart () {
          this.touchStatus = true
      },
      handleTouchMove (e) {
        //   函数节流
          if(this.touchStatus){
              if(this.timer){
                  clearTimeout(this.timer)
              }
                this.timer = setTimeout(() => {
                const touchY = e.touches[0].clientY - 79
                const index = Math.ceil((touchY - this.startY) / 19)
                if(index >=0 && index <= this.letters.length){
                    this.$emit('change', this.letters[index])
                }
              }, 8)
          }
      },
      handleTouchEnd () {
          this.touchStatus = false

      }
  }
}
</script>
<style lang="stylus" scoped>
@import '../../../assets/styles/var.styl';

.list {
  display: flex;
  flex-direction: column;
  justify-content: center;
  position: absolute;
  top: 1.58rem;
  right: 0;
  bottom: 0;
  width: 0.4rem;
  list-style: none;
}

.item {
  text-align: center;
  line-height: 0.4rem;
  color: $bgColor;
}
</style>
